JPH04104531A - Dis equalizer - Google Patents

Abstract

PURPOSE:To obtain a high dispersion compensating capability and an excellent frequency characteristic by forming a strip line whose substrate thickness is >=70% of the thickness as the upper limit which is determined by a specific formula with respect to the upper limit frequency of an intermediate frequency (IF) signal band. CONSTITUTION:In a wavelength dispersion equalizer used in an IF band circuit of a receiver for coherent optical communication, a strip line 1 is formed whose substrate thickness is >= 70% of the thickness as the upper limit determined by a formula I (C is the light velocity and Cr is the dielectric constant of the substrate) with respect to an upper limit frequency fc of the IF signal band, and this strip line 1 is used as the signal propagation line. Thus, the dispersion equalizer is realized which has the dispersion compensating capability higher than conventional and has an excellent frequency characteristic.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a dispersion equalizer that compensates for the effects of chromatic dispersion of optical fibers used in coherent optical communication systems.

(Prior Art) Recent advances in optical fiber amplifiers have brought about the possibility of long-distance optical communications ranging from several 11,000 km to 110,000 km, and the coherent optical communication system is an advantageous system in such multi-stage optical amplification repeater transmission systems. One thing has been pointed out. The reason for this is that the coherent optical communication method has superior frequency selectivity compared to the conventional intensity modulation direct detection method, which makes it possible to receive signals by removing spontaneous emission optical noise generated from optical amplifiers. This is because it can be done. This shows that it is possible to extend the optical repeater interval, improve system reliability, and reduce costs.

(Study on long-distance coherent optical communication systems using direct optical amplifiers, Institute of Electronics, Information and Communication Engineers Optical Communication Systems Study Group 0C889-59) By the way, in long-distance transmission systems aiming at 110,000K, ordinary optical fibers When using a zero-dispersion fiber, the influence of chromatic dispersion is large, so it is essential to use a zero-dispersion fiber.

However, in reality, the signal light wavelength rarely matches the zero-dispersion wavelength, and it is difficult to suppress the influence of dispersion. Now, due to the deviation from the zero dispersion wavelength, lps/km
/nm, the total dispersion of 110000k reaches 10000ps/nm, so even if the modulation speed of the signal light is 2.5Gb/s, CPFSK (
In the optical heterodyne detection method (phase continuous frequency shift modulation), dispersion degradation occurs. As a means to avoid this dispersion degradation, a method has been proposed in which electrical dispersion equalization is performed in the IF band of an optical heterodyne receiver. (K
, Iwashita et al., Chromat.
ic dispersion compensation
in coherent optical comm
``IEEE J-LT Vol.
8. No. 3.1990) This method electrically equalizes the dispersion of the fiber by inserting into the IF signal band a strip line several tens of centimeters in length that has a dispersion characteristic opposite to that of the optical fiber.

(Problem to be solved by the invention) However, the dispersion equalizer reported so far has a length of 0.63 m.
A 50Ω strip line is formed on an alumina substrate with a thickness of m, and the output is 1500 ps/nm at 20 cm.
Since the compensation ability was only moderate, equalizers had to be connected in multiple stages in the above-mentioned high dispersion transmission system, which caused problems such as an increase in circuit scale and deterioration of IF band characteristics. Therefore, an object of the present application is to eliminate such drawbacks and to obtain a dispersion equalizer that is small in size and exhibits little IF band deterioration.

(Means for Solving the Problems) (1) The present invention provides a strip in which the thickness corresponding to the upper limit frequency fc of the IF signal band is set as the upper limit, and the thickness of the substrate is set to 70% or more of this thickness. It forms a line.

(2) In the present invention, a stripline having a characteristic impedance of less than 50Ω is formed on an alumina substrate.

(Function) The characteristic impedance z0 and effective permittivity ε of the microstrip line are given by the following formula. Let the stripline width be W, the thickness of the substrate be h, and the dielectric constant of the substrate be ε. When 1>1, the stripline type dispersion equalizer takes advantage of the fact that there is dispersion in this ε, so it is necessary to introduce the following equation in addition to these equations.

G=0.6+0.0092° However, f is the signal frequency (GHz).

On the other hand, when the effective permittivity dispersion ε' is given, the group delay of the stripline is given by the following equation.

However, e Nistrip line length C: speed of light 0 By substituting the formula ■■■ into the formula, the design formula for the dispersion equalizer can be found. Using this design formula, there are two ways to increase τ when e is constant: 1) Thicken the substrate while keeping zo constant 2) Lower zo.

Among these methods, the method of
If the substrate becomes thicker than /4, higher-order modes will be excited and the transmission characteristics will deteriorate. Therefore, the upper limit of the IF frequency is f
c, the thickness of the board is on the other hand.In method 2), the input and output impedance of high-frequency circuits is usually unified to 50Ω, so a buffer is required to match the impedance, but with 50Ω, the It is possible to achieve high dispersion characteristics that are impossible to achieve.

(Example) FIG. 1 is a diagram showing a first example. This example demonstrates the present invention at a wavelength of 1.5. 10000ps/nm in am band
(75 ps/GHz) dispersion etc. is applied to fixtures. The design conditions are as follows.

e=20cm Er:9.8 (alumina substrate) h=4
mm w=4mm Z. =50Ω f, =6GH
Since the impedance of the strip line 1 in this fixture is 50Ω, a 50Ω coaxial connector 2 can be connected to the input/output port. Therefore, connection with other electric circuits is easy. FIG. 2 is a diagram showing the dispersion equality of this dispersion equalizer. From this figure, it can be seen that the developed dispersion equalizer has a group delay of 75 ps/GHz in the range of 1 to 5 GHz. FIG. 3 shows the frequency characteristics of this dispersion equalizer. Looking at this, it can be seen that good frequency characteristics are obtained in the band up to 6 GHz. As a result, by using the present invention, a small dispersion equalizer having a large dispersion compensation ability of 10,000 ps/nm without deteriorating the frequency characteristics was realized. FIG. 4 shows a second embodiment of the invention. The second embodiment uses the present invention at 2700 ps/mm at a wavelength of 1.5□m band.
(20 ps/GHz) dispersion equalizer. The design conditions are as follows.

(=20cm Er=9.8 (alumina substrate)
h=0.63mm w=2mm Z. =25Ω
This fixture has an alumina substrate with a thickness of 0.63 mm, and a general-purpose substrate can be used as is, so it has the characteristic of reducing costs. However, since the impedance of the strip line 1 is 25Ω, first and second impedance buffer amplifiers 3 and 4 are connected to the input/output ports. The first impedance buffer amplifier 3 has an input impedance of 50Ω and an output impedance of 25Ω.
It is Ω. The second impedance buffer amplifier 4 has an input impedance of 25Ω and an output impedance of 5oΩ. This makes it possible to connect to an external circuit via the 50Ω coaxial connector 2. FIG. 5 is a diagram showing the dispersion characteristics of this dispersion equalizer.

The dispersion equalizer developed from this figure is 20ps/as designed.
It can be seen that it has a group delay of GHz. Figure 6 shows
This dispersion equalizer as well as the conventional dispersion equalizer (6=20cm
, h==0.63mm Sw=0.63mm,
Z. =50Ω.

τ = 1500 ps/nm). Looking at this, it can be seen that the dispersion equalizer according to this embodiment has better frequency characteristics than the conventional one.

As a result, by using the present invention, it was possible to realize a dispersion equalizer having higher dispersion compensation ability and superior frequency characteristics than conventional ones using a conventional alumina substrate.

Modification The first embodiment still has a problem in that it takes time and money to manufacture a thick alumina substrate, but if mass production technology for such substrates advances and the cost becomes low, the substrate can be stripped as is. It is considered practical to produce dispersion equalizers with various values in which only the length of line 1 is changed. Therefore, for example, dispersion compensation of about 1500 ps/nm, which has been reported in the past, can be achieved by using the device according to the present invention with almost no deterioration of frequency characteristics, and the length can also be shortened to about 3 cm.

(Effects of the Invention) By using the present invention, it is possible to realize a small-sized dispersion equalizer that has higher dispersion compensation ability and better frequency characteristics than conventional ones.

[Brief explanation of the drawing]

Fig. 1 shows the first embodiment of the present invention, Fig. 2 shows the dispersion equalization characteristic of the first embodiment, Fig. 3 shows the frequency characteristic of the first embodiment, and Fig. 4 shows the second embodiment of the invention. Embodiment FIG. 5 shows the dispersion equalization characteristics of the second embodiment, and FIG. 6 shows the frequency characteristics of the second embodiment and the conventional dispersion equalizer. In each figure, 1: microstrip line, 2: connector, 3: first impedance buffer amplifier, 4: second impedance buffer amplifier.

Claims (1)

[Claims] 1) In a chromatic dispersion equalizer used in an IF (intermediate frequency) band circuit in a coherent optical communication receiver, C/4f is applied to the upper limit frequency fc of the IF signal band.
A stripline is formed with the thickness determined by c√εr (where C is the speed of light and εr is the dielectric constant of the substrate) and the thickness of the substrate is set to 70% or more of this thickness, and the stripline is A dispersion equalizer characterized in that it is used as a signal propagation path. 2) An optical communication dispersion equalizer characterized in that a stripline with a characteristic impedance of less than 50Ω is formed on an alumina substrate, and the stripline is used as a signal propagation path.